It is well-known that when a thin layer of a metal is electrolytically deposited on an object, the object may be cosmetically improved to attain a lustrous appearance. Such electrolytic deposition can also provide a protective, non-corrosive coating on the object or can be used to strengthen a debilitated object. The process of coating an object by electrolytic deposition of a metal is typically referred to as electroplating.
Included among the various types of metals or metal alloys which can be electrolytically deposited onto an object are bronze, nickel, copper, zinc, silver, platinum and cadmium.
Electroplating essentially involves immersing the object to be plated into an electrolytic bath which is typically composed of, among other things, an aqueous solution of a metal salt such as, for instance, nickel sulfate, copper sulfate, zinc chloride or cadmium oxide, which supplies the quantities of the metal to be deposited. Alternatively, the metal to be plated can be dissolved in an appropriate acid or base, i.e., HCl, H.sub.2 SO.sub.4, HNO.sub.3, KOH or NaOH, and thereafter used to form the electrolytic bath.
Within the electrolytic bath, the object to be plated is usually the cathode and the metal, e.g., the bronze, copper, nickel, zinc or cadmium being deposited, functions as the anode. As such, when an electric current is passed through the electrolytic bath solution, a layer of the desired metal is deposited onto the object which is immersed therein.
The thickness or thinness of the coated layer is proportionate to the amount of time that the object is permitted to remain in the electrolytic bath solution. Thus, if a thicker coating is desired, the object should be permitted to remain in solution for a time period which exceeds that required for a thinner coating. Those skilled in the art are familiar with the variations which occur between processes and apparatuses when a particular thickness of a particular metal is desired.
The rate of deposition of the metal onto the object is a function of the current and voltage applied between the cathode and anode. The rate of deposition between processes and apparatuses for a particular metal and the rate of deposition desired are within the knowledge of those skilled in the art or can be ascertained with little or no experimentation.
Once the object has been coated with a layer of metal, the residual chemicals which remain are rinsed off the electroplated object in some manner in order to prepare the object for subsequent treatment and use. The rinse solution which is used usually consists of water or, after continued use of the process and apparatus, a mixture of water and those chemicals used in the electroplating solution. The concentration of the chemicals in the rinse solution is normally maintained at a concentration which is dilute with respect to the electroplating bath and, usually, substantially more dilute than the concentration of the chemicals in the electroplating bath. The rinse water which remains after rinsing is usually toxic and, as such, must be chemically treated or safely disposed of in some manner.
One known technique for rinsing electroplated objects is generally described in U.S. Pat. Nos. 4,561,956; 3,749,657 and 3,556,970. In each of these disclosures the excess electroplating solution remaining on the coated object is removed by spraying the coated object with a large excess of a rinse solution of water. The effluent which results is typically permitted to drain back into the electroplating tank. Usually there is some treatment of the effluent before it is returned to the electroplating tank.
In another known technique of rinsing electroplated objects, the object is introduced into a plurality of serially arranged rinse tanks to progressively rinse the object. For instance, U.S. Pat. No. 4,379,031 describes an evaporation driven counterflow rinse system and method. The apparatus includes a treatment tank and at least a first and final rinse tank. The treatment tank is adapted to be filled to a predetermined level with a treating solution to which workpieces are submerged for treatment. The first and final rinse tanks are adapted to be filled to a predetermined level with rinsing solution in which the workpieces are submerged to rinse off the treating solution. The rinse solution flows by gravity from the final rinse tank to the first rinse tank. To replace lost rinse water there is provided a rinsing solution inlet, and to return plating chemicals carried from the plating tank by the workpieces, the rinsing solution flows by gravity from the first rinse tank into the plating tank.
U.S. Pat. No. 3,985,628 describes a process for pollution control in electroplating systems. In accordance with the process, a plated object is rinsed by immersing it in a succession of rinse baths contained in rinse tanks. The rinse water from the rinse tanks is then conducted to a scrubber either directly or by way of a collection tank. The rinse water, to which excess heat from the bath has been added, is contacted with an air stream in the scrubber to effect the transfer of the chemical values from the air to the rinse water and to effect the transfer of water and heat from the rinse water to the air. The chemically treated and enriched rinse water is then added to the plating bath.
U.S. Pat. No. 3,928,146 describes an electroplating recovery process. After the objects are plated, they are dipped in the pre-rinse portion of a dual chamber tank, and thereafter passed through a series of rinse tanks to substantially remove all plating chemicals from the objects. The solution in the dual chamber tank contains no more than one-half the amount of chemicals carried over into the rinse tanks. Solution in the various rinse tanks is treated by reverse osmosis to remove chemicals, some solution being returned to other rinse tanks. The concentrate from treatment of the solution in the first rinse is placed in a holding tank. This solution is itself passed through a reverse osmosis unit, the permeate being returned to a rinse tank, the concentrate to the holding tank. Periodically, the contents of the holding tank are returned to the plating bath.
U.S. Pat. No. 3,761,381 describes a plating waste recovery unit wherein the rinse liquid resulting from one of a plurality of wash tubs, into which plated objects are dipped successively to rinse off the plating solution, is circulated through an ion exchange column, where it passes through a resin layer that removes chromic acid and other impurities before the liquid is returned to the wash tub.
U.S. Pat. No. 3,616,437 describes a plating apparatus with recovery of plating chemicals from rinse waters. The apparatus provides a plurality of wash tubs that are connected in series with a plating tank. After the time in which a predetermined number of objects have been plated, the water contained in the tub positioned closest to the plating tank is pumped into a heating and concentrating tower, wherein it is heated and concentrated to the level equivalent to that of the plating solution contained in the plating tank. After such treatment, the water is then transferred into the plating tank.
U.S. Pat. No. 3,542,651 describes a unit for the recovery of plating solution. A plurality of wash tubs are provided into which plated articles are dipped successively to rinse off the plating solution. The water from the tub, into which the articles are first dipped, is sucked into a tower heated by steam to concentrate the tub water to plating strength, and this water is returned to the plating tank.
U.S. Pat. No. 3,663,397 describes a method for the treatment of an electrodeposition bath by controlling the electrodeposition bath composition. Thus, a selective membrane is employed in conjunction with a rinsing process to remove dragout from an electrodeposited object. A portion or all of the rinse material resulting therefrom is returned to the electrodeposition bath.
Finally, U.S. Pat. Nos. 3,663,404 and 3,663,399, each describe a method for treating an electrodeposition bath. The two described methods are similar and essentially include a rinsing step employing, as a rinsing agent, at least a portion of the effluent obtained from a selective separation process, such as an ultrafiltration process.
In addition to the post-plating rinses employed in the electroplating processes and apparatuses of the prior art, there are rinse treatments employed with the pre-plating process steps typically utilized in an electroplating process and apparatus.
One typical pre-plating step is a hot electroclean step. This step comprises immersing the object to be plated in a heated basic or caustic solution, e.g. NaOH or KOH, and passing an electric current therethrough. The hot electroclean bath is typically a "flowing type" bath, i.e. having water flowing in and electroclean solution flowing out. To maintain the composition of the bath, base is added periodically or continuously to the bath.
Once the object to be plated has been electrocleaned, in the usual electroplating process and apparatus the item is immersed in a flowing type rinse bath or series of rinse baths to remove substantially all of the excess base.
After immersion in the post-electroclean rinse bath, the object to be plated is immersed in an acid activator bath. In a zinc plating process, the acid activator bath is usually comprised of a solution of HCl (hydrochloric acid) while in a cadmium plating process, the acid activator bath is usually comprised of a solution of H.sub.2 SO.sub.4 (sulfuric acid). The acid activator bath serves to partly or entirely neutralize any remaining base on the plated object and to activate the surface of the object for subsequent plating.
Once the object to be plated has been immersed in the acid activator bath, the object is usually again immersed in a flowing type rinse bath or series of rinse baths.
The costs associated ,with chemical treatment, disposal and transportation of the various rinse waters to a disposal site are substantial and pose a serious difficulty to an electroplating installation. Another serious difficulty associated with disposing of the rinse water is its negative impact on the environment and on the people who live within the proximity of these waste disposal sites.
Thus, as evidenced by those disclosures discussed hereinabove, which are incorporated herein by reference, there have been attempts to minimize the amounts of contaminated rinse water effluent in electroplating operations.
While these attempts have been partially successful, they often involve huge expenditures of money due to the need to set up a plurality of rinsing tanks.
In addition, because the contaminated rinse water must be treated in some manner before it is returned to the electroplating bath to prevent contamination of the bath, additional costs are associated with this rinse water treatment. Such treatment has necessitated the use of separate rinse water treatment apparatus at electroplating installations.
Accordingly, it is an object and advantage of the present invention to provide an improved process and apparatus for metal electroplating, and especially bronze, nickel, copper, zinc, platinum, silver and cadmium electroplating, which substantially eliminate the necessity to dispose of any waste rinse water resulting from the electroplating process. The process and apparatus of the presently claimed invention is substantially "self-contained", as that term is defined below.
It is another object and advantage of the present invention to provide an improved process and apparatus for metal electroplating, and especially bronze, nickel, copper, zinc, platinum, silver and cadmium plating, which eliminate the need for and costs associated with the treatment and/or disposal of the electroplating rinse water.
It is further object and advantage of the present invention to provide an improved process and apparatus for metal electroplating, and especially bronze, nickel, copper, zinc, platinum, silver and cadmium plating, which eliminate any negative impact that the electroplating waste rinse water has on the environment.